A Kogi truck selling a blend of Korean and Mexican food at Venice beach in Los Angeles. Photographs: Steven Joyce for the Observer

Thomasina Miers stands on a sunny petrol station forecourt in Highland Park, a neighbourhood to the northeast of Los Angeles, enthusiastically eating a marinated beef-tongue taco. "It's so delicious," she says through a mouthful of corn tortilla, coriander and tomatillo salsa.

This low-rise residential area, an hour and a world away from the Walk of Fame, bristles with signs for cheap loans and fast food. It is not on any tourist map. Star-tour buses don't pull up round here. But rather than celebrities, we're looking for Mexican street-food trucks, picking out their vivid orange, blue and pink awnings or distinctive fin-shaped roof vents from among the stucco-covered houses, corner shops and drive-throughs. At the La Estrella and El Pique trucks, we found what we crave. We order more tacos, a beef torta (sandwich) and a buttery hot quesadilla and wolf them as the traffic whizzes past, juices running down our chins.

Miers, former MasterChef winner and co-founder of the Mexican restaurant chain Wahaca, is researching her next venture – a two-year pop-up restaurant opening next month on London's South Bank with a street-food menu. She's here because there are 11 million Mexican-Americans living in California, making up 30% of the population.

"LA has such a huge Mexican population," she says. "In terms of food trends, America, and particularly California, is often five to 10 years ahead of the UK, so it is interesting to see where they're at, particularly with Mexican, which you can find here as easily as getting a hamburger."

Angelenos also have more than 100 years of Mexican street-food history – the arrival and popularity of horse-drawn tamale carts caused the same kind of bureaucratic angst in the 1880s as the influx of taco trucks has done in the past couple of years – so residents have exacting standards. They want rich, slow-cooked sauces, chillies of every hue and heat, soft corn or wheat tortillas piled with meaty fillings, and they want them day and night. There are hundreds of hole-in-the-wall taco stands and trucks to keep them happy.

As we work our way round the best of them, Miers grins from ear to ear. Her interest in Mexican food was piqued when she was in her early 20s and ran a bar in Mexico City. She came home to find she couldn't get authentic Mexican anywhere in the UK and so resolved to open her own casual restaurant. Six years and five Wahacas later, the hunch she and her business partner Mark Selby had about the gap in the market has been proved right and her obsession with Mexico's food has only grown. Yuca's is one of LA's oldest street-food joints – a small shed and plastic awning next to a car park – and she can't help but order almost every dish, while Selby tries to keep things in control.

"I'm on belt hook three at the moment," he says ruefully, "and I'd like to stay that way." Ignoring him, Miers adds a plate of carne asada tacos (grilled beef) to a conchinita pibil (Yucatan-style pork) and machaca (shredded beef) tacos, pickled jalapeño peppers, and a chile verde bean and cheese burrito. This is their first lunch of six today.

Thomasina Miers eating tongue tacos from La Estrella food truck

Our tour continues to take us to corners of Los Angeles most tourists would miss. We visit Olvera Street in Downtown and eat $3 rolled-up beef taquitos with warm avocado sauce from Cielito Lindo. We hit Las Glorias del Buen Comer in Silver Lake – home to an impressive collection of plastic floristry – for creamy shrimp enchiladas in a green coriander sauce; chilaquiles made with fried stale tortillas, salsa and scrambled egg; and huge poblano chillies stuffed with soft white cheese.

At Grand Central Market we watch tripe being packed into split gorditas, deep-fried maize-dough pockets, and are given carnitas (pork tacos) to try by a group of men leaning on a red Formica lunch counter, swigging coke out of huge old glass bottles. The sawdust-covered walkways lead to stalls selling jar after jar of dried chillies and piles of tomatillos (a fruit resembling a green tomato) and jicama, which look like turnips and taste like water chestnut.

The best dishes of the day are served in Guisado's, a small, plain shop in Boyle Heights, a Latino neighbourhood east of Downtown. After we order most of his menu, owner Armando de la Torre takes us to the grocery shop next door where, incongruously, he gets his corn cooked, skinned, hoppered and ground, and then hand-makes it into tortillas or adds lard to make masa for tamales (stuffed steamed breads) or conchas (sweet pastries).

His daughter Natalie loads our table with cardboard plates of tacos topped with tinga de pollo (spiced shredded chicken), skirt steak simmered in pimento sauce, fiery grilled fish with chilli diablo, and a chicken mole (a traditional sauce that takes at least a day to make properly). We try chicharron, made with black beans and slow-cooked pork scratchings. It's much more appetising than it sounds: the fatty pork rinds render down and what's left is melt-in-the-mouth pigginess.

Only one dish defeats us: a chilli taco with habanero ketchup so spicy that it leaves anyone who tastes it red-faced and gasping. Legend has it one visitor managed four in one go. "Eat one, get another free!" says Armando cheerfully as he pours us a shot each of smooth tequila royale reposado to sip. We are, finally, sated.

Next day, the main aim is to find a Kogi truck. Kogi shot to local and then national fame in 2008 when the first of a fleet of five vans popped up selling a surprisingly successful blend of Korean and Mexican food – a signature dish is soy-marinated short rib tacos with coriander and cabbage – and has since spawned dozens of imitators. Its trucks change location up to three times daily and can only be located via Twitter, where Kogi has 96,000 followers; in 2010 its head chef, Roy Choi, was named best new chef in the US by Food & Wine magazine – quite an accolade for a kitchen the size of a caravan.

We find the truck (and its lengthy queue) outside an office block next to the Fox studios in Century City and feast on kimchi quesadillas, which are sweet, gingery cheese and pickled cabbage tortilla sandwiches, and Kogi sliders: miniature brioche buns with Korean bulgogi-marinated short-rib burgers. We finish the meal with an entirely unnecessary and ridiculously moreish home-made chocolate bar with squidgy chilli salt caramel and peanuts.

"Coming here, you can see authentic Mexican cooking and lots of its famous street food, but you can also see where else this food can go," says Miers. "This is some of the most exciting, innovative food we've tried. This trip is giving me masses of ideas to work with!"

Santa Monica beach, the usual pull for Los Angeles tourists

Half an hour later we pull up at Komodo, another Korean-Mexican-Asian-Californian truck, where somehow find room in our stomachs to try delicate fish and grape tacos with pickled cucumber and sesame oil and pork meatballs that are charred and crisp on the outside and pink and lightly spiced inside. Although they're incredibly tasty, even Miers agrees that ordering two portions of truffled potato fries was a mistake; we attempt to cleanse our palates with lychee juice.

Our final stop of the day is Mariscos Chente, a seafood restaurant where we sample drunken shrimp and chipotle prawns and drink vast micheladas – beer mixed with Clamato (clam and tomato) juice and lime, served in a chilli-salt rimmed glass. Then it's back to the hotel, where we swim lengths of the pool in a bid to make room for dinner.

Oddly, Los Angeles's very high-end Mexican restaurants don't seem to deliver food that's as exciting as the city's street food. In the evenings we try two of the best: Border Grill, founded by Mary Sue Milliken and Susan Feniger, and Red O, which belongs to Rick Bayless. These three chefs have done more than anyone to popularise and legitimise Mexican food in America. But although the meals were fine, it was as though the punchy mouthfuls of flavour we'd been ploughing through all day didn't taste the same inside, off pieces of fine china and eaten with cutlery; they didn't give us the same intense sense of the place and its inhabitants as the foods they cook and eat every day could. However good a posh restaurant's heirloom tomato tostadito or yellowtail ceviche might be, they couldn't beat those tongue tacos eaten in the roar of the traffic.•Thomasina Miers' new recipe book Wahaca – Mexican Food at Home is out next month

How to do itPreheat the oven to 130C/gas mark 1. Cut the pork belly into six roughly equal pieces. Rub with salt and leave to sit for 1 hour. Put the pork chunks into a large casserole pan with the rest of the ingredients and bring to simmering point. Cover well with a tight-fitting lid or foil and cook in the oven for 2-3 hours until the pork is so soft it can be cut with a spoon.Scoop out the pieces with a slotted spoon and arrange them on a baking sheet. Turn the oven up to 190C/gas mark 5. Roast the pork for about 30 minutes, until the pieces are crispy, golden and caramelised. Roughly chop the meat and serve on a wooden board or heated plate with bowls of coriander, white onion or shallot, wedges of lime, a salsa, and warm tortillas for making your own tacos.